process for preparing 1,6-diacetoxy-2,5-dimethyl-2.hexene; 1,2,6-triacetoxy-2,5-dimethyl-hexane; and 2,5-dimethyl-tetra-hydropyran-2-methanol acetate



United States Patent 3,124,599 PROCESS FOR PREPARING 1,6-DIACETOXY-2.,S-

DIMETHYL-Z-HEXENE; 1,2,6 TRIACETOXY-2,5--

4 Claims. Cl. 260345.8)

This invention relates to novel processes for the prep aration of 2,idimethyl-ZA-hexadiene. More particularly it relates to methods for preparing 2,5-.dimethyl-2,4- hexadiene from methacrolein and certain novel intermediates obtained thereby.

This application is a divisional application of Serial No. 641,235, now Patent No. 2,910,520, H. R. Guest et a1. entitled Process for Preparing 2,5-Dimethyl-2,4-Hexadione, filed February 20, 1957 [and assigned to the same assignee as the present invention.

Heretofore it ha been suggested to prepare 2,5-dimethyl-2,4-hexadiene by treating methallyl chloride with magnesium dissolved in a suitable medium such as ethyl ether to form 2,S-dimethyl-1,5-hexadiene. This product was then converted to 2,5-dimethyl-2,4-hexad-iene by treatment with a suitable acid catalyst in liquid phase or chrome alumina, for example, in vapor phase.

The reaction of magnesium in ethyl ether with methallyl chloride or the reaction of metallic sodium with isocrotyl chloride is, however, a hazardous procedure when undertaken on a scale suitable for commercial purposes. A further difficulty exists in that the expensive metals employed were converted to the chlorides which have virtually no value.

We have now discovered, unexpectedly, that 2,5-dimethyl-2,4-hexadiene can be prepared by alternative processes 'from methacrolein, a relatively inexpensive starting material. These processes involve a series of steps, the reactions of which are described as follows:

PROCESS 1 Step 1 Methacrolein is caused to react by heating at temperatures of approximately 140 C. to 225 C. and preferably at 150 C. and under autogenous pressure e.g., 500 pounds per square inch (p.s.i.) to form methacrolein dimer (3,4 dihydro 2,5adimethyl-2-tormyl-2H-ipyran), which when hydrogenated with Raney nickel catalyst at temperatures of approximately 80 C. to 160 C. and under pressure of 100 pounds per square inch (p.s.i.) to 1000 p.s.i. forms 2,5-dimethyltetrahydropyran-Z metha- Patented Mar. 10., 1964 nol. It should be noted that a preferred hydrogenation temperature of C. with a pressure of approximately p.s.i. This reaction can be described graphically in the following manner.

methaerolein CH3 CH 3,4-dihydro-2,5dimethyl- 2-forrnyl-2H-pyran (methacrolein dimer) H2 filmy Nickel o H; o H,

2,5-dimethyltetrahydropyran-zmethanol Step 2 A novel compound, 1,6-diacetoxy 2,S-dimethyl-Z-hexene is prepared from 1,2,6-triacetoxy-2,S-dimethylhexane by the release of acetic acid therefrom. ;1,2,6-triacetoxy-2,5- dirnethylhexane is prepared directly by the treatment of 2,5 -dimethyltetrahydropyran-Z-methanol from Step 1, with acetic anhydride and a Lewis acid or strong mineral acid such as, for example, aluminum chloride, boron trifluoride, zinc chloride, stannic chloride, sulfuric acid, phosphorous pentoxide, nitric acid or phosphoric acid as a catalyst therefore at elevated temperatures, e.g., 140 C. to C. Further novel compounds resulting from this same reaction in small quantitative amounts are 2,5-dimethyltetrahydropyran-Z-methanol acetate and l,2,6-triacetoxy-2,5- dimethylhexane. Further reaction of these latter compounds by further heating with acetic anhydride and a Lewis acid or strong mineral acid will result in their conversion to the desired diester, 1,6-diacetoxy-2,S-dimethyl- 2-hexane. This is illustrated by the following schematic equation:

O H ll HC-O-C-OH; H ll 0 HGO-C--OH g C CHa CHsC-O- -C11s acetic A H anhydride H2 i CHaCOOH CH2 catalyst CH 140160 O. aceticacid H OH: 0

CH3 H O I g -OCOH C-O CHs H; 2

1,6-diacetoxy- 1,2,6-triacet0xy- 2,5-dimethyl-2- 2,5-dimethylhexane hexene acetic anhydrirle catalyst 1 I 140 (IL-160 0. CH3 (353 CH3 CH;

C CH/gCH; CH oH/zpHa H2 E g O CHzOH 0 CHnO-GCHa 2,5-dimethyl- 2,5-dimethyltetra O tetrahydropyranhydropyran-2-methanol 2-methan0l acetate $1 3 \acetic anhydride '50 catalyst 140 G.160 O. a:

a o e E2 o- 0 ii 0H3 3 H3O- I O--iZOH 5 E HaO(|3H O C-O-(i-OHa 2 1,2,6-triacetoxy- 2,5-dimethylhexane p 3 Step 4 The 1,6-diacetoxy-2,S-dimethyl-Z-hexene from Step 2 is then hydrogenated With a Raney nickel catalyst under autogenous pressure e.g., 150 p.s.i. at C. to 65 C. and preferably C. This reaction results in the formulation of about sixty percent by weight of l,6-diacetoxy-2,5-dimethylhexane, a novel compound. In the remainder hydrogenolysis of one of the ester groups of the starting material, 1,6-diacetoxy-2,5-dimethy1-2-hexene, occurs with the formation of acetic acid and the novel unsaturated monoester, 2,5-dimethyl-4-hexeny1 acetate. Under the condition of the reaction the resultant double bond is not hydrogenated.

This reaction can be illustrated graphically as follows:

2,5-dimethyl-2,4-hexadiene. 'which is the 1,5-isomer, referred to hereinabove, can be Both products resulting from Step 3 are pyrolyzed to form a hexadiene mixture, i.e., 2,5-dirnethyl-2,4-hexadiene and 2,5-dimethyl-1,5-hexadiene. The preferred thermal treatment temperature is about 335 C. This pyrolysis is accomplished by passing the filtered product from the hydrogenation of Step 3 over a suitable aluminum-conconverted to the desired 2,5-dimethyl-2,4-hexadiene by vapor-phase isomerization over a chrome-alumina catalyst. This isomerization can be accomplished alternatively by heating the 1,5-diene in the liquid phase with a catalytic amount of a strong mineral acid, such as, preferably, sulfuric acid, para-toluenesulfonic acid, benzenesulfonic acid or sulfoacetic acid. Less preferred strong mineral acids Which can be employed are, for example, phosphoric acid, nitric acid, and hydrochloric acid.

6 hereinabove. The methacrolein dimer can be hydrogenated at a pressure of 2500 to 5000 p.s.i. or preferably 4000 p.s.i. and temperatures of 140 C. to 350 C. using CH3 CH3 CH C =CHGH JHCH OO a copper chromium catalyst to form 2,5-dimethyl-1,6

5 a 2,5 dimethy1 4 hexeny1acetate hexanediol a novel compound. Alternat vely, the dimer can be hydrogenated at a temperature In the range of 3 (313a E 40 C. to 160 C. and preferably about 100 C. using a CHaCOOCH OHCH CHZOHOH;OOCCH: E Raney nickel catalyst at pressure of about 100 to 1000 G o 1fidmcetoxyajdimethfl hexane p.s.1. With a preferred pressure of about 150 p.s.i., to form 2,5-dimethyl tetrahydropyran-Z-methanol, subsequent hy- CH CH3 g d'rogenolysis of which, at a temperature of about 225 C. CHrzcHCH:(IJ CH2 CHQOOOH to 400 C., or preferably 300 C, and a pressure of 2500 2 5 th 12 4h 1 T2 to 5000 p.s.i., or again a preferred pressure of 4000 p.s.i. me y exadlene acetic ac d N and employing a copper-chromium-barium catalyst results C 3 3 1 in the formation of the diol, i.e., 2,5-dimethyl-1,6-hexane- CH3=COH2CHg-C=CH ZOHQC'OOH diol, a novel compound. Acetylation of this diol causes 2 fi dim thyl l fi h dien acemacid the production of l,6-diacetoXy-2,S-dimethylheXane. This Isomerization hexane can then be pyrolyzed as described above in Step 11 OH: Vapor Phase 4 of Process 1 to form substantially the same product mix- CH2:(|3 CHzCH2 =CH2 E 0PM Catalyst ture of 2,5-dimethyl-2,4-hexadiene, 2,S-dimethyl-1,5- hex- 2 5 th 11 i q gg r adiene and acetic acid described therein. The diene mixme y- 'fi-hexadlene m lqmd age ture is se arated from the acetic acid produced in the CH3 CH3 p 1 reaction by a standard steam distillation, and the 1,5- OHTC=OE"OH=G GH diene product converted as described in Step 4 of Process 2,5'dimethY1-214'hexadiene 1 to the desired 2,4-diene isomer. The preparation of PROCESS 2 1,6-diacetoxy-Z,S-dimethylhexane, pyrolysis of which results in the diene mixture referred to immediately herein- A novel alternate. method for the preparation of 2,5 i dimethyl-2,4-hexadiene involves the synthesis of methabove, can be illustrated graphically as follows:

on CH CHaO a 140 o.-225 0. can on, on;

H A togeneous HO CHO pressure O 0 CH0 methaerolein dimer -1000 p.s.i. 2500 to 5000p.s.i. Hi 80 0.400 0.1-1, -350 C. Raney N1 Ou-Or OH 2500-5000 OH H E? CH on 0H OH OH CH cfi on CH;

3- 2 H00 2 z 2- a Cu-Cr-Ba 2,5-dimethyl-L6-hexanediol 225 O.-400 O. H:

0 OH OH 2,5-dimethy1tetrahydropyran-2-methanol CH3 CH3 acrolein dimer (3,4 dihydro-Z,S-dimethyl-Z-fiormyl-ZH- pyran) as described in Step 1 of Process 1 referred to acetic anhydride 2,5-dimethyl-1,6hexanediol diacetate (1,G-diacetoxy-2,5dimethylhexane) CHaCOOH acetic acid 2,S-dimethyl-Z,4-hexadiene, prepared from the novel products of this invention, is an important starting material in the synthesis of chrysanthemum acid chloride from which pyrethrin, a valuable insecticide, can be synthesized. The series of steps necessary to this prepara tion may be illustrated schematically as follows:

The preparation of pyrethrin from chrysanthemum acid chloride is described by LaForge and Barthel in the Journal of Organic Chemistry, vol. 12, pages 199 to 202 (1947).

It should be noted, in addition, that 2,5-dimethyl-1,6- hexanediol is also of value in the preparation of esters which are of value as plasticizers, solvents and synthetic lubricants, and is of particular value when employed as an insect repellant.

The following examples are further illustrative of our invention:

EXAMPLE 1 (a) A charge of 1000 grams (g.) of methacrolein (92 percent) and 10 grams of hydroquinone was placed in a 3.1 stainless steel Adkins bomb and heated to 140 C. for 16 hours while agitating by means of a rocking mechanism in which the bomb was suspended. The initial pressure was 195 p.s.i. and fell to 150 p.s.i. at the conclusion of the run.

The material was discharged after cooling and the unreacted methacrolein stripped off at atmospheric pressure. The distillation was continued at reduced pressure and 744 grams of 3,4-dihydro-2,S-dimethyl-Z-formyl- ZH-pyran boiling at 50 C. at 10 mm. was obtained. A high boiling residue remained which weighed 99 grams. The yield to the dimer was 81 percent with an efiiciency of 86 percent based on the methacrolein charged.

(b) A charge of 1000 grams of methacrolein dimer prepared in the foregoing manner and 50 grams of commercial Raney nickel catalyst were placed in an Adkins bomb contained in a rocking mechanism and hydrogenated at 150 C. and 2000 p.s.i. hydrogen pressure. The material was discharged, filtered, and distilled. A total of 914 grams of 2,5-dimethyltetrahydropyran-Z- methanol boiling at 51 C./ 1.1 mm. was obtained. This was a yield of 91 percent based on the dimer charged.

A charge of 4000 grams of acetic anhydride, 576 grams of 2,5-dimethyltetrahydropyran-Z-methanol and 8 grams of zinc chloride were placed in a distilling flask and refluxed for 11 hours at a kettle temperature of 140 C. At the end of the reflux period the acetic acid produced in the reaction and the excess acetic anhydride were distilled off to a head temperature of 69 C./4 mm. A small amount of 2,S-dimethyltetrahydropyran- 2-methanol acetate was collected as a heads out and 743 grams of 1,6-diacetoxy-2,5-dimethyl-2-hexene boiling over the range of C./2 mm. to C./1.0 was obtained as v.the main fraction. A small tails cut containing 1,2,6-triacetoxy-2,S-dirnethylhexane was obtained. The total weight of the two by-product esters was 88 grams. These materials are converted to the desired unsaturated diester with good eiilciency, 85 percent, by further reaction with acetic anhydride in the presence of the catalyst as described immediately above.

The yield. of the desired product was 81.3 percent and the efficiency 89.9 precent based on the amount of methacrolein charged.

The novel intermediate products prepared by this acetlyation reaction were characterized as follows:

1,2,6-triacetoxy-2,S-dimethylhexane:

Boiling Point (B.P.) C.- C./2

mm. Specific Gravity (Sp. Gr.) at

20/20 C 1.064. Index of refraction (N 30/D)- 1.4370. 2,5 -dimethyltetrahydropyran-Z-methanol acetate:

B.P 60 C./1 mm.

Sp. Gr. at 20/20 C 0.995.

N30/D a 1.4420.

1,6-diacetoxy-2,S-dimethyl-Z-hexenez B.P 105 C.-110 C./2

Sp. Gr. at 20/20 C 1.010.

2,5-dimethyl-4-hexenyl acetate:

B.P 38 C./0.lmn1.

Sp. Gr. at 20/20 C 0.878.

N30/D 1.4214. 1,6-diacetoxy-2,5-dimethylhexane:

B.P 101 C./1 mm.

Sp. Gr. at 20/20 C 0.986.

(e) The apparatus employed for this step in the process was a steel tube 1 inch in diameter (I.D.) fitted with an electrically-wound jacket containing an oil by means of which the temperature could be maintained at a constant level. The tube was charged with 340 cc. of a catalyst in the following manner:

To 375 grams of activated alumina pellets (4 X 8 mesh) was added a solution of 450 grams of Al(NO .9H O in 300 cc. of water. This mixture was allowed to reflux gently for 2 hours. The liquid was then drained from the catalyst particles. A solution of 138 grams of diam- 5 monium phosphate in 300 cc. of water was then added a kettle temperature of 52 C./ 25 mm. A total of 588 grams of distillate was obtained .while a high-boiling residue of 15 grams remained behind. The distillate was then charged to a distilling flask and distilled with water. The top layer was removed as distillate while the lower layer was returned to the kettle. A total of 378 grams of upper layer Was obtained while 301 grams of material which contained 60.3 percent acetic acid remained in the kettle. The distillate was then fractionated and 296 grams of mixed 2,5-dimethyl-1,5-hexadiene and 2,5-dimethyl-2,4-hexadiene was obtained. This represented a yield of 91.9 percent of the theoretical. Approximately 60 percent of the mixture of dienes was the desired 2,4 isomer.

The catalyst for the isomerization was prepared as follows:

Activated alumina pellets (500 grams of 4 x 8 mesh) were shaken with a solution of 50 grams of chromic acid in 250 cc. of water for 20 minutes. The excess liquid was drained off and the catalyst was dried overnight at 120 C.

The reactor was a stainless steel tube 1% inches diam. and 4 feet in length jacketed with an electrically-heated reservoir containing a heating oil. Five hundred cc. of catalyst prepared in the above manner was charged to the lower section of the reactor and the upper part was filled with 750 cc. of 4 x 8 mesh Aloxite (a commercial variety of fused aluminum oxide). A fiow of hydrogen was passed through the tube While the temperature was held at 425 C. for 5 hours.

A total of 886 grams of 2,5-dimethyl-1,5-hexadiene was then fed at the rate of 90 cc. per hour while the temperature was held at 220-225 C. This provided a contact time of 72 seconds for the vapors in the catalyst zone.

The gas effiuent from the converter was condensed by passing through a water cooled condenser and a total of 882 grams were obtained. This was found to be 90 percent 2,5-dimethyl-2,4-hexadiene. The yield based on the 1,5 isomer fed was 89.5 percent with an efficiency of 95.7 percent.

A charge of 300' grams of 2,5-dimethyl-l,5-hexadiene, 1.5 grams of para-toluenesulfonic acid, and 0.5 gram tert-butyl catechol was placed in a flash equipped with a reflux condenser and heated for 3 /2 hours at a kettle temperature of 135 C. The product was distilled under reduced pressure and 250 grams of the desired 2,5- dimethyl-2,4-hexadiene was obtained along with 40 grams of unchanged 1,5 diene. The yield to the 2,4-isomer was therefore 83.2 percent and the efficiency 96.3 percent.

EXAMPLE 2 (a) 1. A mixture of 873 grams of 2,5-dimethyltetrahydropyran-Z-methanol and 25 grams of barium promoted copper chromite catalyst (Harshaw Chemical Company Cu0 4011) was hydrogenated at 300 C. and 4000 pounds per square inch pressure for 6 hours. The reaction product was filtered and distilled under reduced pressure to obtain 2,5dimethyl-1,6-hexanediol having these properties: distillation range 117122 C./ 2.5 mm., Sp. Gr. 20/20 0.950, N 30/D 1.4550, molecular weight by the Menzies-Wright method 148.5 (theoretical 146), equivalent weight by hydroxyl analysis 73.4 (theoretical 73). The yield and efficiency were 45 percent and 83 percent respectively. This colorless, viscous diol, which is a new compound, was approximately 5 percent soluble in water at 20 C.

(a) 2. A mixture of 1000 grams of methacrolein dimer and 30 grams of copper chromite catalyst (Harshaw Chemical Company Cu-0102-P) was stirred in a 1 gallon autoclave under 3000 p.s.i. of hydrogen pressure at 140 C. After 30 minutes, the absorption of hydrogen had ceased. The temperature and pressure were increased to 300 C.350 C. and 4000 p.s.i. to complete the reaction. After 5.5 hours under these conditions the product was 10 cooled, filtered and distilled. The 2,5-dimethyl-1,6-hexanediol was obtained with 25 percent yield and 78 percent efficiency.

(a) 3. A mixture of 1000 grams of methacrolein dimer and 50 grams of Raney nickel was hydrogenated in a rocking, stainless steel bomb at 1500 p.s.i. pressure and 78 C.-l C. The temperature was then increased to 295 C.-3'00 C. with a pressure of 3500 p.s.i. for 5 hours. The product was filtered and distilled to obtain 2,S-dimethyl-1,6-hexanediol with a 10 percent yield. The 2,5-dimethyltetrahydropyran-Z-methanol was formed with 85 percent yield.

(a) 4. A mixture of 1964 grams of 2,5-dimethyltetrahydropyran-2-methanol and 60 grams of copper-iron chromite was hydrogenated in an autoclave at 300 C.- 305 C. and 4000 p.s.i. for 6 hours. The product was filtered and distilled to obtain 2,5-dimethyl-1,6-hexanediol with 12 percent yield.

([1) While 568 grams of 2,5-dimethyl-1,6-hexanediol (3.89 moles) were heated at 140 C.160 C., 1588 grams of acetic anhydride (15.56 moles, percent excess) were added over a period of 30 minutes. The acetic acid was then distilled to a head temperature of 134 C., and a kettle temperature of 186 C. The residue was fractionated under reduced pressure to obtain the diacetate having the following properties: Boiling Point 101 C. at 1 mm., N 30/!) 1.4292, Sp. Gr. 20/20 C. 0.986, observed molecular weight 220 (theoretical 230), observed equivalent weight 118 (theoretical The yield and efliciency were 99 percent based on the diol. This colorless, water insoluble diacetate is a new compound.

(c) A 1 inch stainless steel tube 3 feet long was packed with 450 cc. of 4 x 8 mesh activated alumina catalyst. While the catalyst was heated at 375 C., the diacetate was fed at the rate of 50 cc. per hour. The product made from 228 grams of ester was fractionated under reduced pressure to obtain 91 grams of mixed 2,5-dimethylhexadienes. Analysis of the dimethylhexadiene fraction showed that 60 percent of it was 2,5-dimethyl-2,4-hexadiene and the balance was 2,5-dimethyl-1,5-hexadiene. The overall yield and efficiency to the dimethylhexadienes was 83 percent. These 2,5-dimethylhexadienes were separated readily by either fractional distillation or by fractional crystallization. Since the 2,5-dirnethyl-1,5-hexadiene is isomerized to the desired 2,5-dimethyl-2,4-hexadiene by passing over activated alumina catalyst or by an acid catalyst in the liquid phase, the former diene may be recycled with the diacetate to the vapor phase reaction tube to produce more 2,S-dimethyl2,4-hexadiene.

What is claimed is:

1. A process for preparing a mixture of 1,6-diacetoxy- 2,5-dimethyl-2-hexene; 1,2,6-triacetoxy-2,S-dimethylhexane; and 2,S-dimethyltetrahydropyran-Z-methanol acetate which comprises forming a mixture of 2,5-dimethyltetrahydropyran2-methanol, acetic anhydride and a catalyst selected from the group consisting of a Lewis acid and a strong mineral acid and heating said mixture at a temperature of C. to C.

2. A process for preparing 1,6-diacetoxy-2,S-dimethyl- 2-hexene which comprises forming a mixture of 2,5-dimethyltetrahydropyran-Z-methanol, acetic anhydride and a catalyst selected from the group consisting of a Lewis acid and a strong mineral acid and heating said mixture at a temperature of 140 C. to 160 C.

3. A process for preparing 1,6-diacetoxy-2,S-dimethyl- 2-hexene which comprises forming a mixture of 1,2,6-triacetoxy-2,S-dirnethylhexane, acetic anhydride and a catalyst selected from the group consisting of a Lewis acid and a strong mineral acid and heating said mixture at a temperature of 140 C. to 160 C.

4. A process for preparing 1,6-diacetoxy-2,S-dimethyl- 2-hexene which comprises forming a mixture of 2,5-dimethyltetrahydropyran-2-methanol acetate, acetic anhydride and a catalyst selected from the group consisting 11 12 of a Lewis acid and a strong mineral acid and heating said 2,852,531 Hoffman Sept. 16, 1958 mixture at a temperature of 140 C. to 160 C. 2,910,520 Guest et a1 Oct. 27, 1959 OTHER REFERENCES References Cited in the file of this patent C B 1 H db h 4th Ed 1 2 2 d arnere: e1 stems an uc v0 n UNITED STATES PATENTS 6 suppfipage 158 (1943) 2,610,193 Whetstone et a1 P' 1952 Simon et al.: Helv. Chim. Acta., v01. 29, pages 1133 2,773,895 Ballard et a1 Dec- 11, 1956 44 (1946). 

1. A PROCESS FOR PREPARING A MIXTURE OF 1,6-DIACETOXY2,5-DIMETHYL-2-HEXENE; 1,2,6-TRIACETOXY-2,5-DIMETHYLHEXANE; AND 2,5-DIMETHYLTETRAHYDROPYRAN-2-METHANOL ACETATE WHICH COMPRISES FORMING AMIXTURE OF 2,5-DIMETHYLTETRAHYDROPYRAN-2-METHANOL, ACETIC ANHYDRIDE AND A CATALYST SELECTED FROM THE GROUP CONSISTING OF A LEWIS ACID AND A STRONG MINERAL ACID AND HEATING SAID MIXTURE AT A TEMPERATURE OF 140*C. TO 160*C. 